The invention relates to a process for manufacturing solid cathodes for a fused salt electrolytic cell for aluminum production, such that at least the working faces are wettable by the precipitated metal.
In the electrolytic production of aluminum from aluminum oxide the latter is dissolved in a fluoride melt comprised for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the molten aluminum itself or a solid body wettable by aluminum acting as the cathode. Attached to the anode beam and dipping into the electrolyte from above are anodes which in conventional processes are made of amporphous carbon. As a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced at the anodes, combining with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process generally takes place in the temperature range 940.degree.-970.degree. C.
It is known that when large current densities are employed the combination of vertical components of the resultant magnetic field with the horizontal components of current lead to an undesireable deformation or doming of the surface of the some centimeter deep metal bath and also to undesired, pronounced flow or stirring of the metal. At small interpolar distances this undesired deformation of the metal bath can be so large that the aluminum touches the anodes and causes short circuiting.
Furthermore, the flowing action of the metal leads to increased chemical dissolution of the metal at the surface or to a fine dispersion of the aluminum in the electrolyte; as a result of re-oxidation this leads to a lower current yield.
A lower current density would in principle be of advantage, but would involve unacceptable higher capital costs for the cells and the pot room.
Known for some time now are solid cathodes which are wettable by aluminum and permit a much smaller interpolar distance. The precipitated aluminum forms a film of metal on these solid cathodes which are such that the aluminum continuously flows off them. Another development is such that per projected base area of anode a plurality of solid cathodes; spaced apart from each other and having relatively small working faces; project out of the aluminum sump. These cathode bodies are mostly made of titanium diboride, which is a very expensive material. For this reason application on an industrial scale has not succeeded up to now.
The high cost of cathodes made form pure titanium diboride arises from the following three process steps:
(1) Titanium diboride is produced at high temperature by carbothermic means or plasma technology.
(2) Very fine grinding is necessary for powder production.
(3) Because of the need to produce green-strength blocks and the difficulty of furnace operation, a high reject rate results during shaping and sintering; energy and capital costs are high.
In principle there are two ways of lowering the high material costs for titanium diboride:
(1) Titanium diboride is deposited as a coating on a cheaper substrate, and/or
(2) the titanium diboride is diluted with a cheaper material but at most to such an extent that the cathode as a whole is still wet by aluminum and still conducts electricity well.
Described in the German Patent Publication DE-DS 23 05 281 is a cathode or cathode element for the fused salt electrolytic production of aluminum, which is such that it has on one face a layer made up from a binery system comprising a refractory material and a small fraction of carbon, the said system being an eutectic of hard refractory metal and carbon. One of the starting materials, however, is very expensive titanium diboride, which limits the economic use of the cathode elements.
Described in the U.S. Pat. Nos. 3,661,736 and 4,308,114 is a solid cathode for fused salt electrolytic production of aluminum made up of a composite material. Refractory granules of a material which can be wet by aluminum are embedded in a carbon matrix. The production of the composite material involves, according to the first of these patents, mixing a fine carbon powder with granular titanium diboride and treating the mixture in a suitable thermal process; according to the second of these patents granular titanium diboride is mixed into tar and pitch. Such cathodes made of a composite material are much less resistant than cathodes made of pure titanium diboride as the carbon matrix comes into contact with the molten electrolyte.
Further, described in No. WO 82/01018 is a cathode made of a titanium diboride/graphite composite which always contains a high concentration of carbon. From the low TiB.sub.2 content disclosed in this publication the expert can conclude that in that case either there is no titanium diboride/carbon eutectic present at all, or it is possibly present as a dispersion in a carbon matrix. It is clear that subsequent impregnation of the porous composite is necessary in order to achieve adequate mechanical strength.
The object of the present invention is to develop a process for manufacturing wettable solid cathodes for the fused salt electrolytic production of aluminum, which is such that it leads to substantially less expensive products of constant, good quality.